There is a family of devices that are based on thermo-optically tunable, thin-film optical filters. These devices, which are made from amorphous semiconductor materials, exploit what had previously been viewed as an undesirable property of amorphous silicon, namely, its large thermo-optic coefficient. The performance of these devices is based on trying to maximize thermo-optic tunability in thin-film interference structures, instead of trying to minimize it as is often the objective for conventional fixed filters. The devices are characterized by a pass band centered at a wavelength that is controlled by the temperature of the device. In other words, by changing the temperature of the device one can shift the location of the pass band back and forth over a range of wavelengths and thereby control the wavelength of the light that is permitted to pass through (or be reflected by) the device.
The basic structure for the thermo-optically tunable thin film filter is a single cavity Fabry-Perot type filter 10, as illustrated in FIG. 1a. The Fabry-Perot cavity includes a pair of thin film multi-layer interference mirrors 14a and 14b separated by a spacer 16. The thin film mirrors are made up of alternating quarter wave pairs of high and low index films. The two materials that are used for the layers are a-Si:H (n=3.67) and non-stoichiometric SiNx (n=1.77). In addition the spacer (“cavity”) also is made of amorphous silicon. To produce more complex pass band characteristics or more well defined pass bands, multiple cavities can be concatenated to form a multi-cavity structure.
To achieve control over the temperature of the device, at least some embodiments include a ZnO or polysilicon heater film 12 integrated into the multilayer structure. The heater film is both electrically conductive and optically transparent at the wavelength of interest (e.g. 1550 nm). Thus, by controlling the current that is passed through the film, one can control the temperature of the filter.
The thermal tuning that is achievable by this thermo-optically tunable filter is illustrated by FIG. 1b. The configuration used an amorphous silicon spacer with dielectric mirrors (tantalum pentoxide high index and silicon dioxide low index layers, deposited by ion-assisted sputtering, R=98.5% mirror reflectivity). That structure was heated in an oven from 25 C to 229 C. The tuning was approximately 15 nm or dλ/dT=0.08 nm/K.